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1
Put oven rack in middle position and preheat oven to 375F.
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2
Butter a shallow 2-quart baking dish.
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3
Cook onion in 1 1/2 tablespoons butter with a pinch of salt in a 12-inch heavy skillet over moderately low heat, covered, stirring occasionally, until softened, about 5 minutes.
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4
Increase heat to moderately high and add mushrooms, then saute, stirring occasionally, until mushrooms begin to give off liquid, about 2 minutes.
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5
Add soy sauce and continue to saute mushrooms, stirring, until liquid mushrooms give off is evaporated.
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6
Add Sherry and boil, stirring occasionally, until evaporated.
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7
Remove from heat.
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8
Melt remaining 3 tablespoons butter in a 2- to 3-quart heavy saucepan over moderately low heat and whisk in flour, then cook roux, whisking, 3 minutes.
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9
Add broth in a stream, whisking, and bring to a boil, whisking.
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10
Whisk in milk and simmer sauce, whisking occasionally, 5 minutes.
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11
Stir in mushroom mixture, lemon juice, and salt.
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12
Flake tuna into sauce and stir gently.
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13
Season sauce with salt and pepper.
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14
Cook noodles in a 5- to 6-quart pot of boiling salted water until al dente.
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15
Drain noodles in a colander and return to pot.
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16
Add sauce and stir gently to combine.
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17
Transfer mixture to baking dish, spreading evenly.
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18
Toss together bread crumbs and cheese in a bowl.
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19
Drizzle with oil and toss again, then sprinkle evenly over casserole.
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20
Bake until topping is crisp and sauce is bubbling, 20 to 30 minutes.
